The present invention relates to a communication device, in which a power supply line for supplying electric power to a receiver circuit is used for transmitting a reception signal of the receiver circuit.
A conventional inter-vehicle communication device mounted in a vehicle transmits and receives electrical beacon signals and optical beacon signals to communicate with road-side devices provided along travel roads, as disclosed in the following patent document 1. In this inter-vehicle communication device, a transmitter/receiver circuit for transmitting and receiving the electrical beacon signal and the optical beacon signal is positioned on a dashboard in a vehicle, for example, so that electric wave and light are easily transmitted and received. Thus, the transmitter/receiver circuit is formed separately from a main circuit for processing transmission and reception signal of the electrical beacon and the optical beacon.
For simplifying wiring in the vehicle, the transmitter/receiver circuit and the main circuit are connected via a single electrical line thereby to not only supply electric power from the main circuit to the transmitter/receiver circuit and transmit and receive the transmission and reception signal of the electrical beacon and the optical beacon by multiplexing.
Patent document 1: JP 10-256992A
The electrical beacon signal and the optical beacon signal are used to supply various road traffic information to vehicles. The electrical beacon signal and the optical beacon signal are transmitted to a navigation device so that the received road traffic information is provided to passengers by way of map display or voice announcement in the vehicle.
More and more navigation devices recently use a thin liquid crystal display panel as a display device and light emitting diodes as a backlight source of the liquid crystal display panel. The light emitting diode is used, because it is more advantageous than a cold cathode tube in respect of environmental protection, low voltage driving and low temperature operation characteristics.
The luminance of the backlight source (brightness of the liquid crystal display panel) is normally adjustable by a user in accordance with changes in brightness of the surrounding area between day and night. For example, the light emitting diodes used as the backlight source are connected in series with a switching element so that supply of current to the light emitting diodes are turned on and off by turning on and off the switching element. The light emitting diodes are driven to emit light in a desired luminance by controlling the duty ratio of a pulse-width modulation (PWM) signal, which turns on and off the switching element connected in series with the light emitting diodes.
When the current supply to the light emitting diodes is turned from on to off or from off to on, the input voltage supplied to the light emitting diodes is changed correspondingly by the change in the current supply. Thus, light modulation noise develops on a power supply line. Such noise on the power supply line influences other devices such as a transmitter/receiver circuit, which is connected to the same power supply line.
In the optical beacon, in which the power voltage and the reception signal are multiplexed on the single signal line, the light modulation noise on the single signal line will in some cases cause loss of a part of the reception signal received from an external site when the reception signal is transmitted to succeeding circuit stages in the communication device. As a result, the road traffic information cannot be acquired properly.